Cellular and Synaptic Architecture of Multisensory Integration in the Mouse Neocortex

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Sound-driven modulation of sub- and suprathreshold activity in mouse primary visual cortex

Integration of multimodal information is essential for the integrative response of the brain and is thought to be accomplished mostly in sensory association areas. However, available evidences in humans and monkeys indicate that this process begins already in primary sensory cortices. However, how cross-modal synaptic integration occurs in vivo along cortical microcircuitries remains to be investigated. Primary sensory cortices of rodents are well-suited to address this issue as they have a well-known anatomy and synaptic physiology. Here we quantified how acoustic stimulation (white noise, 50 ms duration) affects spontaneous and sensory-driven activity of pyramidal neurons in different layers of primary visual cortex by intrinsic signal imaging-targeted in vivo whole-cell recordings in lightly anesthetized and awake, head-fixed mice. Acoustic stimuli reliably evoked hyperpolarizations -lasting about 200-300 ms- in layer 2-3 and 6 neurons, but not in the main thalamorecipient lamina, layer 4. We found depolarizing responses to sound only in layer 5 (about 1/4 of recorded neurons), whereas the remaining cells exhibited no response or hyperpolarizations. To explore the synaptic nature of sound-driven hyperpolarizations in supragranular pyramids, we measured the inhibitory and excitatory conductances elicited by sound. Hyperpolarizations were due to the combined effect of activation of inhibitory conductances along with a withdrawal of excitatory ones. In agreement with this, sound-driven hyperpolarizations were significantly reduced by intracellular perfusion with a cesium-based solution containing 1 mM picrotoxin to block GABA-B and GABA-A receptors, respectively (-3,3 ± 0,4 mV vs -1,1 ± 0,3 mV, t-test, p<0,001). We next quantified the impact of sound-driven inhibition on visual responsiveness by coupling flashed or moving light bars with white noise. Sub- and suprathreshold responses were significantly reduced in the case of bimodal stimulation compared to the pure visual modality (of about 30 and 50%, respectively, paired statistics, p<0.05). Finally, transection experiments guided by intrinsic signal imaging indicated that auditory-driven synaptic inputs onto visual cortical neurons persisted despite inactivation of horizontal connections between primary visual and auditory cortices. Taken together, our findings illustrate a simple scheme by which spontaneous and evoked activity in a retinotopic column of the mouse primary visual cortex can be shaped by the acoustic external environment in a layer-specific manner

Structure and flexibility in cortical representations of odour space

The cortex organizes sensory information to enable discrimination and generalization1-4. As systematic representations of chemical odour space have not yet been described in the olfactory cortex, it remains unclear how odour relationships are encoded to place chemically distinct but similar odours, such as lemon and orange, into perceptual categories, such as citrus5-7. Here, by combining chemoinformatics and multiphoton imaging in the mouse, we show that both the piriform cortex and its sensory inputs from the olfactory bulb represent chemical odour relationships through correlated patterns of activity. However, cortical odour codes differ from those in the bulb: cortex more strongly clusters together representations for related odours, selectively rewrites pairwise odour relationships, and better matches odour perception. The bulb-to-cortex transformation depends on the associative network originating within the piriform cortex, and can be reshaped by passive odour experience. Thus, cortex actively builds a structured representation of chemical odour space that highlights odour relationships; this representation is similar across individuals but remains plastic, suggesting a means through which the olfactory system can assign related odour cues to common and yet personalized percepts

Il silenzio è d'oro. Depressione delle connessioni orizzontali peririnali indotta dalla memoria di riconoscimento visivo.

La memoria di riconoscimento visivo è una delle forme più semplici di apprendimento nei mammiferi. L’incapacità di discriminare la familiarità d’un oggetto e d’una persona è uno dei primi sintomi della malattia di Alzheimer. Recenti studi d’ablazione e farmacologici nella scimmia e nei roditori hanno dimostrato che la corteccia peririnale, una struttura del sistema medio-temporale, svolge un ruolo necessario per la discriminazione visiva della familiarità d’un oggetto. I neuroni della corteccia perinale vanno incontro ad una caratteristica riduzione della risposta quando l’animale incontra per la seconda volta un oggetto familiare. Negli ultimi anni, è stato dimostrato che la corteccia peririnale può andare incontro a plasticità sinaptica in vitro. E’ stato dunque proposto che la discriminazione di familiarità, segnalata dalla riduzione di risposta delle cellule peririnali, sia mediata da fenomeni simili alla depressione a lungo termine (LTD). Ad oggi, però, nessuno studio ha ancora verificato quest’ipotesi. La componente peririnale della memoria di riconoscimento visivo è basata su meccanismi LTD-like? La mia tesi tenta di rispondere a questa domanda cruciale. Per questo scopo ho messo a punto tre nuovi protocolli non ancora descritti in letteratura: 1) Familiarizzazione visiva distribuita nel topo con oggetti bidimensionali (overtraining); 2) Induzione elettrica di LTD NMDA-dipendente dei potenziali di campo lungo le connessioni orizzontali dello strato II/III della corteccia peririnale del topo (paired pulses low frequency stimulation, PPLFS); 3) Occlusione della plasticità sinaptica peririnale indotta mediante overtraining e valutata ex vivo attraverso induzioni ripetute di LTD attività-dipendente (con PPLFS) o di LTP attività-dipendente (con theta-bursts) a due ore dall’ultima sessione di familiarizzazione. I miei risultati indicano: 1) Una riduzione dell’efficacia sinaptica lungo le connessioni orizzontali dello strato II/III della corteccia peririnale nei topi che hanno incontrato oggetti durante l’overtraining (familiarizzati) rispetto ai topi che, sottoposti allo stesso protocollo comportamentale, non hanno, però, incontrato oggetti nell’apparato (controlli); 2) Un’occlusione parziale dell’LTD NMDA-dipendente nei topi familiarizzati rispetto ai controlli; 3) Cambiamenti dell’LTP complementari a quelli dell’LTD. Questa tesi, dunque, dimostra per la prima volta che la memoria di riconoscimento visivo ottenuta con familiarizzazioni distribuite sfrutta meccanismi simili all’LTD lungo le connessioni orizzontali dello strato II/III della corteccia peririnale del topo

Preserved Excitatory-Inhibitory Balance of Cortical Synaptic Inputs following Deprived Eye Stimulation after a Saturating Period of Monocular Deprivation in Rats

Monocular deprivation (MD) during development leads to a dramatic loss of responsiveness through the deprived eye in primary visual cortical neurons, and to degraded spatial vision (amblyopia) in all species tested so far, including rodents. Such loss of responsiveness is accompanied since the beginning by a decreased excitatory drive from the thalamo-cortical inputs. However, in the thalamorecipient layer 4, inhibitory interneurons are initially unaffected by MD and their synapses onto pyramidal cells potentiate. It remains controversial whether ocular dominance plasticity similarly or differentially affects the excitatory and inhibitory synaptic conductances driven by visual stimulation of the deprived eye and impinging onto visual cortical pyramids, after a saturating period of MD. To address this issue, we isolated visually-driven excitatory and inhibitory conductances by in vivo whole-cell recordings from layer 4 regular-spiking neurons in the primary visual cortex (V1) of juvenile rats. We found that a saturating period of MD comparably reduced visually-driven excitatory and inhibitory conductances driven by visual stimulation of the deprived eye. Also, the excitatory and inhibitory conductances underlying the synaptic responses driven by the ipsilateral, left open eye were similarly potentiated compared to controls. Multiunit recordings in layer 4 followed by spike sorting indicated that the suprathreshold loss of responsiveness and the MD-driven ocular preference shifts were similar for narrow spiking, putative inhibitory neurons and broad spiking, putative excitatory neurons. Thus, by the time the plastic response has reached a plateau, inhibitory circuits adjust to preserve the normal balance between excitation and inhibition in the cortical network of the main thalamorecipient layer

MD does not alter the ratio of excitation and inhibition measured at the excitation peak.

<p>The values of the ratio between g_E and g_I measured at the peak of the excitatory conductance are plotted for each experimental group. MD does not significantly modify the ratio values for both contralateral (left) and ipsilateral (right) eye responses (Mann-Whitney Rank Sum tests, p>0.6).</p

Estimate of visually-driven excitatory and inhibitory synaptic conductances.

<p><b>A</b>. Visual responses to an optimally oriented moving light bar of a 4RSN recorded under 1 mM QX314 while injecting different steady currents. The black, continuous line are the recorded V_m values, whereas the blue, dashed trace shows the reconstructed V_m values obtained by inserting back the estimated g_E and g_I values into the fundamental membrane equation. The instantaneous total synaptic conductance is calculated based on the instantaneous slope of the current-voltage relation (G_tot, blue). The time-dependent excitatory (g_E, green) and inhibitory (g_I, red) conductances are plotted below. Gray traces represent the 95% confidence intervals obtained by bootstrapping of the data (see Methods). Conductance measurements began after the response to the injected current was at steady state (after 200 ms). Resting conductances were calculated in absence of visual stimulation (dashed line: stimulus end). <b>B</b>. Visually-driven PSPs vary linearly with the injected current. Plot showing the linearity of the relationship between the amplitude of the visually-driven PSP response and the value of the injected current (r = −0.97) for a 4RSN (this plot refers to the example shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0082044#pone-0082044-g002" target="_blank">Figure 2</a> of the Main Text). Means ± standard erros are shown. The median of the correlation coefficients for all the recorded neurons was −0.94 (25^th–75^th percentiles: −0.88 − −0.99). <b>C</b>. Plot of the recorded <i>vs</i> reconstructed V_m values obtained by inserting back the estimated g_E and g_I into the membrane equation. The linearity of the cell and the accuracy of the V_m reconstruction is shown by the fact that data points align along the line of steepness 1 and intercept 0 in the plot. <b>D</b>. Temporal intervals between the peaks of g_E (green) and g_I (red) and that of the V_m response. For each cell, values for both contralateral and ipislateral responses are plotted. Note that in the vast majority of cases the conductance values have been obtained in close proximity of the V_m peak response (within 200 ms, dashed lines). <b>E</b>. Example of a voltage clamp recording (see Methods) following visual stimulation with a moving bar in the preferred direction. By clamping the cell at the reverse potential for inhibition (−80 mV when considering a liquid junction potential of approximately 14 mV) only excitatory currents can be seen (green: average response overlapped to single trials, shown in gray). Conversely, clamping the cell at the reversal potential for inhibition (+14 mV when considering the liquid junction potential) reveals the presence of inhibitory currents (red: average response overlapped to single trials, shown in gray).</p

A saturating period of MD reduces both excitatory and inhibitory responses to closed eye stimulation.

<p><b>A</b>. Examples of excitatory (g_E, red) and inhibitory (g_I, green) responses of 4RSNs (left: normal; right: MD) upon stimulation with optimally oriented light bars. The total membrane conductance (G_tot) is shown in blue and the V_m response in absence of current injection is in black (top traces). Dotted lines: 0 nS. Gray shadow: 95% confidence intervals of the g_E and g_I estimates obtained by bootstrap analysis. <b>B</b>. Amplitudes of the visually-driven g_E (green) and g_I (red) responses in normal (open boxes) and MD (dashed boxes) rats. MD reduced both excitatory and inhibitory conductances upon contralateral eye (closed in MD rats) stimulation and increased both excitatory and inhibitory conductances upon ipsilateral eye (open in MD rats) stimulation (Mann-Whitney Rank Sum tests, p<0.05). <b>C</b>. The excitatory-inhibitory balance of visually-driven responses, expressed by the K_EI index, was not affected by MD (dashed boxes vs open boxes) for both contralateral and ipsilateral responses (Mann-Whitney Rank Sum tests, p>0.7).</p

Basic biophysical properties of 4RSNs.

<p>± S.E.M. are given. No significant differences between Control and MD rats for all parameters (t-tests, p>0.2). Means </p

Effects of offsetting equilibrium potentials V_E and V_I on g_E and g_I estimates.

<p>^th–75^th percentiles). All comparisons along any column between normal (N) and MD rats were significant (Mann-Whitney Rank Sum tests, p<0.05). V_E and V_I are the estimated equilibrium potentials for excitation and inhibition (see methods section), V_E+ and V_I+ are V_E and V_I offset by +10 mV; V_E− and V_I− are V_E and V_I offset by −10 mV. Data are reported as follows: median (25</p